is not communicated to the atmosphere by radiation, yet the air is
warmed by contact with heated bodies, in the same manner as solids or
liquids. The stratum of air which is immediately in contact with the
earth is heated by it; it becomes specifically lighter and rises, making
way for another stratum of air which is in its turn heated and carried
upwards; and thus each successive stratum of air is warmed by coming in
contact with the earth. You may perceive this effect in a sultry day, if
you attentively observe the strata of air near the surface of the earth;
they appear in constant agitation, for though it is true the air is
itself invisible, yet the sun shining on the vapours floating in it,
render them visible, like the amber dust in the water. The temperature
of the surface of the earth is therefore the source from whence the
atmosphere derives its heat, though it is communicated neither by
radiation, nor transmitted from one particle of it to another by the
conducting power; but every particle of air must come in contact with
the earth in order to receive heat from it.
EMILY.
Wind then by agitating the air should contribute to cool the earth and
warm the atmosphere, by bringing a more rapid succession of fresh strata
of air in contact with the earth, and yet in general wind feels cooler
than still air?
MRS. B.
Because the agitation of the air carries off heat from the surface of
our bodies more rapidly than still air, by occasioning a greater number
of points of contact in a given time.
EMILY.
Since it is from the earth and not the sun that the atmosphere receives
its heat, I no longer wonder that elevated regions should be colder than
plains and valleys; it was always a subject of astonishment to me, that
in ascending a mountain and approaching the sun, the air became colder
instead of being more heated.
MRS. B.
At the distance of about a hundred million of miles, which we are from
the sun, the approach of a few thousand feet makes no sensible
difference, whilst it produces a very considerable effect with regard to
the warming the atmosphere at the surface of the earth.
CAROLINE.
Yet as the warm air rises from the earth and the cold air descends to
it, I should have supposed that heat would have accumulated in the upper
regions of the atmosphere, and that we should have felt the air warmer
as we ascended?
MRS. B.
The atmosphere, you know, diminishes in density, and consequently in
weight, as it is more distant from the earth; the warm air, therefore,
rises only till it meets with a stratum of air of its own density; and
it will not ascend into the upper regions of the atmosphere until all
the parts beneath have been previously heated. The length of summer even
in warm climates does not heat the air sufficiently to melt the snow
which has accumulated during the winter on very high mountains, although
they are almost constantly exposed to the heat of the sun’s rays, being
too much elevated to be often enveloped in clouds.
EMILY.
These explanations are very satisfactory; but allow me to ask you one
more question respecting the increased levity of heated liquids. You
said that when water was heated over the fire, the particles at the
bottom of the vessel ascended as soon as heated, in consequence of their
specific levity: why does not the same effect continue when the water
boils, and is converted into steam? and why does the steam rise from the
surface, instead of the bottom of the liquid?
MRS. B.
The steam or vapour does ascend from the bottom, though it seems to
arise from the surface of the liquid. We shall boil some water in this
Florence flask, (PLATE IV. Fig. 1.) in order that you may be well
acquainted with the process of ebullition;--you will then see, through
the glass, that the vapour rises in bubbles from the bottom. We shall
make it boil by means of a lamp, which is more convenient for this
purpose than the chimney fire.
[Illustration: Plate IV. Vol. I. p. 84.
Fig. 1. Pneumatic Pump.
Ether evaporated & water frozen in the air pump.
A Phial of Ether.
B Glass vessel containing water.
C.C Thermometers one in the Ether, the other in the water.
Fig. 2. Boiling water in a flask over a Patent lamp.]
EMILY.
I see some small bubbles ascend, and a great many appear all over the
inside of the flask; does the water begin to boil already?
MRS. B.
No; what you now see are bubbles of air, which were either dissolved in
the water, or attached to the inner surface of the flask, and which,
being rarefied by the heat, ascend in the water.
EMILY.
But the heat which rarefies the air inclosed in the water must rarefy
the water at the same time; therefore, if it could remain stationary in
the water when both were cold, I do not understand why it should not
when both are equally heated?
MRS. B.
Air being much less dense than water, is more easily rarefied; the
former, therefore, expands to a great extent, whilst the latter
continues to occupy nearly the same space; for water dilates
comparatively but very little without changing its state and becoming
vapour. Now that the water in the flask begins to boil, observe what
large bubbles rise from the bottom of it.
EMILY.
I see them perfectly; but I wonder that they have sufficient power to
force themselves through the water.
CAROLINE.
They _must_ rise, you know, from their specific levity.
MRS. B.
You are right, Caroline; but vapour has not in all liquids (when brought
to the degree of vaporization) the power of overcoming the pressure of
the less heated surface. Metals, for instance, mercury excepted,
evaporate only from the surface; therefore no vapour will ascend from
them till the degree of heat which is necessary to form it has reached
the surface; that is to say, till the whole of the liquid is brought to
a state of ebullition.
EMILY.
I have observed that steam, immediately issuing from the spout of a
teakettle, is less visible than at a further distance from it; yet it
must be more dense when it first evaporates, than when it begins to
diffuse itself in the air.
MRS. B.
When the steam is first formed, it is so perfectly dissolved by caloric,
as to be invisible. In order however to understand this, it will be
necessary for me to enter into some explanation respecting the nature of
SOLUTION. Solution takes place whenever a body is melted in a fluid. In
this operation the body is reduced to such a minute state of division by
the fluid, as to become invisible in it, and to partake of its fluidity;
but in common solutions this happens without any decomposition, the body
being only divided into its integrant particles by the fluid in which it
is melted.
CAROLINE.
It is then a mode of destroying the attraction of aggregation.
MRS. B.
Undoubtedly. --The two principal solvent fluids are _water_, and
_caloric_. You may have observed that if you melt salt in water, it
totally disappears, and the water remains clear, and transparent as
before; yet though the union of these two bodies appears so perfect, it
is not produced by any chemical combination; both the salt and the water
remain unchanged; and if you were to separate them by evaporating the
latter, you would find the salt in the same state as before.
EMILY.
I suppose that water is a solvent for solid bodies, and caloric for
liquids?
MRS. B.
Liquids of course can only be converted into vapour by caloric. But the
solvent power of this agent is not at all confined to that class of
bodies; a great variety of solid substances are dissolved by heat: thus
metals, which are insoluble in water, can be dissolved by intense heat,
being first fused or converted into a liquid, and then rarefied into an
invisible vapour. Many other bodies, such as salt, gums, &c. yield to
either of these solvents.
CAROLINE.
And that, no doubt, is the reason why hot water will melt them so much
better than cold water?
MRS. B.
It is so. Caloric may, indeed, be considered as having, in every
instance, some share in the solution of a body by water, since water,
however low its temperature may be, always contains more or less
caloric.
EMILY.
Then, perhaps, water owes its solvent power merely to the caloric
contained in it?
MRS. B.
That, probably, would be carrying the speculation too far; I should
rather think that water and caloric unite their efforts to dissolve a
body, and that the difficulty or facility of effecting this, depend both
on the degree of attraction of aggregation to be overcome, and on the
arrangement of the particles which are more or less disposed to be
divided and penetrated by the solvent.
EMILY.
But have not all liquids the same solvent power as water?
MRS. B.
The solvent power of other liquids varies according to their nature, and
that of the substances submitted to their action. Most of these
solvents, indeed, differ essentially from water, as they do not merely
separate the integrant particles of the bodies which they dissolve, but
attack their constituent principles by the power of chemical attraction,
thus producing a true decomposition. These more complicated operations
we must consider in another place, and confine our attention at present
to the solutions by water and caloric.
CAROLINE.
But there are a variety of substances which, when dissolved in water,
make it thick and muddy, and destroy its transparency.
MRS. B.
In this case it is not a solution, but simply a mixture. I shall show
you the difference between a solution and a mixture, by putting some
common salt into one glass of water, and some powder of chalk into
another; both these substances are white, but their effect on the water
will be very different.
CAROLINE.
Very different indeed! The salt entirely disappears and leaves the water
transparent, whilst the chalk changes it into an opaque liquid like
milk.
EMILY.
And would lumps of chalk and salt produce similar effects on water?
MRS. B.
Yes, but not so rapidly; salt is, indeed, soon melted though in a lump;
but chalk, which does not mix so readily with water, would require a
much greater length of time; I therefore preferred showing you the
experiment with both substances reduced to powder, which does not in any
respect alter their nature, but facilitates the operation merely by
presenting a greater quantity of surface to the water.
I must not forget to mention a very curious circumstance respecting
solutions, which is, that a fluid is not nearly so much increased in
bulk by holding a body in solution, as it would by mere mixture with the
body.
CAROLINE.
That seems impossible; for two bodies cannot exist together in the same
space.
MRS. B.
Two bodies may, by condensation, occupy less space when in union than
when separate, and this I can show you by an easy experiment.
This phial, which contains some salt, I shall fill with water, pouring
it in quickly, so as not to dissolve much of the salt; and when it is
quite full I cork it. --If I now shake the phial till the salt is
dissolved, you will observe that it is no longer full.
CAROLINE.
I shall try to add a little more salt. --But now, you see, Mrs. B., the
water runs over.
MRS. B.
Yes; but observe that the last quantity of salt you put in remains solid
at the bottom, and displaces the water; for it has already melted all
the salt it is capable of holding in solution. This is called the point
of _saturation_; and the water in this case is said to be _saturated_
with salt.
EMILY.
I think I now understand the solution of a solid body by water
perfectly: but I have not so clear an idea of the solution of a liquid
by caloric.
MRS. B.
It is probably of a similar nature; but as caloric is an invisible
fluid, its action as a solvent is not so obvious as that of water.
Caloric, we may conceive, dissolves water, and converts it into vapour
by the same process as water dissolves salt; that is to say, the
particles of water are so minutely divided by the caloric as to become
invisible. Thus, you are now enabled to understand why the vapour of
boiling water, when it first issues from the spout of a kettle, is
invisible; it is so, because it is then completely dissolved by caloric.
But the air with which it comes in contact, being much colder than the
vapour, the latter yields to it a quantity of its caloric. The particles
of vapour being thus in a great measure deprived of their solvent,
gradually collect, and become visible in the form of steam, which is
water in a state of imperfect solution; and if you were further to
deprive it of its caloric, it would return to its original liquid state.
CAROLINE.
That I understand very well. If you hold a cold plate over a tea-urn,
the steam issuing from it will be immediately converted into drops of
water by parting with its caloric to the plate; but in what state is the
steam, when it becomes invisible by being diffused in the air?
MRS. B.
It is not merely diffused, but is again dissolved by the air.
EMILY.
The air, then, has a solvent power, like water and caloric?
MRS. B.
This was formerly believed to be the case. But it appears from more
recent enquiries that the solvent power of the atmosphere depends solely
upon the caloric contained in it. Sometimes the watery vapour diffused
in the atmosphere is but imperfectly dissolved, as is the case in the
formation of clouds and fogs; but if it gets into a region sufficiently
warm, it becomes perfectly invisible.
EMILY.
Can any water dissolve in the atmosphere without its being previously
converted into vapour by boiling?
MRS. B.
Unquestionably; and this constitutes the difference between
_vaporization_ and _evaporation_. Water, when heated to the boiling
point, can no longer exist in the form of water, and must necessarily be
converted into vapour or steam, whatever may be the state and
temperature of the surrounding medium; this is called vaporization. But
the atmosphere, by means of the caloric it contains, can take up a
certain portion of water at any temperature, and hold it in a state of
solution. This is simply evaporation. Thus the atmosphere is continually
carrying off moisture from the surface of the earth, until it is
saturated with it.
CAROLINE.
That is the case, no doubt, when we feel the atmosphere damp.
MRS. B.
On the contrary, when the moisture is well dissolved it occasions no
humidity: it is only when in a state of imperfect solution and floating
in the atmosphere, in the form of watery vapour, that it produces
dampness. This happens more frequently in winter than in summer; for the
lower the temperature of the atmosphere, the less water it can dissolve;
and in reality it never contains so much moisture as in a dry hot
summer’s day.
CAROLINE.
You astonish me! But why, then, is the air so dry in frosty weather,
when its temperature is at the lowest?
EMILY.
This, I conjecture, proceeds not so much from the moisture being
dissolved, as from its being frozen; is not that the case?
MRS. B.
It is; and the freezing of the watery vapour which the atmospheric heat
could not dissolve, produces what is called a hoar frost; for the
particles descend in freezing, and attach themselves to whatever they
meet with on the surface of the earth.
The tendency of free caloric to an equilibrium, together with its
solvent power, are likewise connected with the phenomena of rain, of
dew, &c. When moist air of a certain temperature happens to pass through
a colder region of the atmosphere, it parts with a portion of its heat
to the surrounding air; the quantity of caloric, therefore, which served
to keep the water in a state of vapour, being diminished, the watery
particles approach each other, and form themselves into drops of water,
which being heavier than the atmosphere, descend to the earth. There are
also other circumstances, and particularly the variation in the weight
of the atmosphere, which may contribute to the formation of rain. This,
however, is an intricate subject, into which we cannot more fully enter
at present.
EMILY.
In what manner do you account for the formation of dew?
MRS. B.
Dew is a deposition of watery particles or minute drops from the
atmosphere, precipitated by the coolness of the evening.
CAROLINE.
This precipitation is owing, I suppose, to the cooling of the
atmosphere, which prevents its retaining so great a quantity of watery
vapour in solution as during the heat of the day.
MRS. B.
Such was, from time immemorial, the generally received opinion
respecting the cause of dew; but it has been very recently proved by a
course of ingenious experiments of Dr. Wells, that the deposition of dew
is produced by the cooling of the surface of the earth, which he has
shown to take place previously to the cooling of the atmosphere; for on
examining the temperature of a plot of grass just before the dew-fall,
he found that it was considerably colder than the air a few feet above
it, from which the dew was shortly after precipitated.
EMILY.
But why should the earth cool in the evening sooner than the atmosphere?
MRS. B.
Because it parts with its heat more readily than the air; the earth is
an excellent radiator of caloric, whilst the atmosphere does not possess
that property, at least in any sensible degree. Towards evening,
therefore, when the solar heat declines, and when after sunset it
entirely ceases, the earth rapidly cools by radiating heat towards the
skies; whilst the air has no means of parting with its heat but by
coming into contact with the cooled surface of the earth, to which it
communicates its caloric. Its solvent power being thus reduced, it is
unable to retain so large a portion of watery vapour, and deposits those
pearly drops which we call dew.
EMILY.
If this be the cause of dew, we need not be apprehensive of receiving
any injury from it; for it can be deposited only on surfaces that are
colder than the atmosphere, which is never the case with our bodies.
MRS. B.
Very true; yet I would not advise you for this reason to be too
confident of escaping all the ill effects which may arise from exposure
to the dew; for it may be deposited on your clothes, and chill you
afterwards by its evaporation from them. Besides, whenever the dew is
copious, there is a chill in the atmosphere which it is not always safe
to encounter.
CAROLINE.
Wind, then, must promote the deposition of dew, by bringing a more rapid
succession of particles of air in contact with the earth, just as it
promotes the cooling of the earth and warming of the atmosphere during
the heat of the day?
MRS. B.
Yes; provided the wind be unattended with clouds, for these
accumulations of moisture not only prevent the free radiation of the
earth towards the upper regions, but themselves radiate towards the
earth; under these circumstances much less dew is formed than on fine
clear nights, when the radiation of the earth passes without obstacle
through the atmosphere to the distant regions of space, whence it
receives no caloric in exchange. The dew continues to be deposited
during the night, and is generally most abundant towards morning, when
the contrast between the temperature of the earth and that of the air is
greatest. After sunrise the equilibrium of temperature between these two
bodies is gradually restored by the solar rays passing freely through
the atmosphere to the earth; and later in the morning the temperature of
the earth gains the ascendency, and gives out caloric to the air by
contact, in the same manner as it receives it from the air during the
night. --Can you tell me, now, why a bottle of wine taken fresh from the
cellar (in summer particularly), will soon be covered with dew; and even
the glasses into which the wine is poured will be moistened with a
similar vapour?
EMILY.
The bottle being colder than the surrounding air, must absorb caloric
from it; the moisture therefore which that air contained becomes
visible, and forms the dew which is deposited on the bottle.
MRS. B.
Very well, Emily. Now, Caroline, can you inform me why, in a warm room,
or close carriage, the contrary effect takes place; that is to say, that
the inside of the windows is covered with vapour?
CAROLINE.
I have heard that it proceeds from the breath of those within the room
or the carriage; and I suppose it is occasioned by the windows which,
being colder than the breath, deprive it of part of its caloric, and by
this means convert it into watery vapour.
MRS. B.
You have both explained it extremely well. Bodies attract dew in
proportion as they are good radiators of caloric, as it is this quality
which reduces their temperature below that of the atmosphere; hence we
find that little or no dew is deposited on rocks, sand, water; while
grass and living vegetables, to which it is so highly beneficial,
attract it in abundance--another remarkable instance of the wise and
bountiful dispensations of Providence.
EMILY.
And we may again observe it in the abundance of dew in summer, and in
hot climates, when its cooling effects are so much required; but I do
not understand what natural cause increases the dew in hot weather?
MRS. B.
The more caloric the earth receives during the day, the more it will
radiate afterwards, and consequently the more rapidly its temperature
will be reduced in the evening, in comparison to that of the atmosphere.
In the West-Indies especially, where the intense heat of the day is
strongly contrasted with the coolness of the evening, the dew is
prodigiously abundant. During a drought, the dew is less plentiful, as
the earth is not sufficiently supplied with moisture to be able to
saturate the atmosphere.
CAROLINE.
I have often observed, Mrs. B., that when I walk out in frosty weather,
with a veil over my face, my breath freezes upon it. Pray what is the
reason of that?
MRS. B.
It is because the cold air immediately seizes on the caloric of your
breath, and, by robbing it of its solvent, reduces it to a denser fluid,
which is the watery vapour that settles on your veil, and there it
continues parting with its caloric till it is brought down to the
temperature of the atmosphere, and assumes the form of ice.
You may, perhaps, have observed that the breath of animals, or rather
the moisture contained in it, is visible in damp weather, or during a
frost. In the former case, the atmosphere being over-saturated with
moisture, can dissolve no more. In the latter, the cold condenses it
into visible vapour; and for the same reason, the steam arising from
water that is warmer than the atmosphere, becomes visible. Have you
never taken notice of the vapour rising from your hands after having
dipped them into warm water?
CAROLINE.
Frequently, especially in frosty weather.
MRS. B.
We have already observed that pressure is an obstacle to evaporation:
there are liquids that contain so great a quantity of caloric, and whose
particles consequently adhere so slightly together, that they may be
rapidly converted into vapour without any elevation of temperature,
merely by taking off the weight of the atmosphere. In such liquids, you
perceive, it is the pressure of the atmosphere alone that connects their
particles, and keeps them in a liquid state.
CAROLINE.
I do not well understand why the particles of such fluids should be
disunited and converted into vapour, without any elevation of
temperature, in spite of the attraction of cohesion.
MRS. B.
It is because the degree of heat at which we usually observe these
fluids is sufficient to overcome their attraction of cohesion. Ether is
of this description; it will boil and be converted into vapour, at the
common temperature of the air, if the pressure of the atmosphere be
taken off.
EMILY.
I thought that ether would evaporate without either the pressure of the
atmosphere being taken away, or heat applied; and that it was for that
reason so necessary to keep it carefully corked up?
MRS. B.
It is true it will evaporate, but without ebullition; what I am now
speaking of is the vaporization of ether, or its conversion into vapour
by boiling. I am going to show you how suddenly the ether in this phial
will be converted into vapour, by means of the air-pump. --Observe with
what rapidity the bubbles ascend, as I take off the pressure of the
atmosphere.
CAROLINE.
It positively boils: how singular to see a liquid boil without heat!
MRS. B.
Now I shall place the phial of ether in this glass, which it nearly
fits, so as to leave only a small space, which I fill with water; and in
this state I put it again under the receiver. (PLATE IV. Fig. 1.)* You
will observe, as I exhaust the air from it, that whilst the ether boils,
the water freezes.